1. FIELD OF THE INVENTION
The present invention relates to an apparatus for selecting terminal equipment connected to telephone lines by detecting the line impedance at an exchange center so as to select either a telephone or a data transmitter at the terminal.
2. DESCRIPTION OF THE PRIOR ARTS
A simple apparatus which performs satisfactorily has not been found. It has been proposed to provide the circuit shown in FIG. 1 which has a device for measuring the line loop impedance in a telephone line which is equipped at the exchange center to detect a variation of line loop impedances by the off-hook operation of the telephone.
In FIG. 1, the terminal device comprises a network control unit (1) (hereinafter referring to as NCU); a data terminal equipment (2) (hereinafter referring to DTE) and a telephone (3). The exchange center comprises a station power source (4); a relay contact (5) for inversion of the line; a reference resistance (6) for measuring a line loop resistance and a control circuit having an analogue-digital converter (7).
The selection of either the telephone (3) or the data terminal equipment (2) is carried out by switching the relay contact (5) for inversion of the line. In FIG. 1, the telephone (3) is selected. When the relay contact (5) is switched, the positive potential is applied in the line L.sub.2 whereby a diode D.sub.1 of NCU (1) is turned on to connect the power source to NCU (1) and DTE (2) so as to make NCU (1) and DTE (2) operable. The telephone (3) can not be used when DTE (2) is used whereas DTE (2) can not be used when the telephone is used. In the apparatus shown in FIG. 1, the telephone (3) is predominantly selected. When the hook switch H.multidot.S is closed by removing the telephone (3) from its hook during the period of data transmission through the lines L.sub.1, L.sub.2 by DTE (2), current is passed through the telephone (3). The telephone current is detected by a control circuit (8) at the exchange center and the telephone (3) is selected by switching the relay switch (5). The relay switch (5) selects the telephone (3) or DTE (2) by inverting DC voltage applied through the lines L.sub.1, L.sub.2.
The operation for predominantly selecting the telephone (3) over DTE (2) will be illustrated.
The situation where NCU (1) at the terminal is actuated to operate DTE (2) and a modulator-demodulator (MODEM) (22) is connected to the lines L.sub.1, L.sub.2 will be considered. The DC resistance from the line to NCU (1) and DTE (2) is set to be greater than the DC resistance of the telephone (3). That is, the line loop current passing through NCU (1) and DTE (2) is less than the current passing through the telephone (3). Thus, when DTE (2) is under operation, the increase of the line loop current caused by the off-hook condition (closing the hook switch HS) of the telephone is detected by the potential between both terminals of the reference resistance (6) using the AD conversion of the analogue-digital converter (7).
In the conventional apparatus, the off-hook operation of the telephone (3) is detected as described. Thus, an AD converter (7) is needed in the exchange center. Moreover, the off-hook condition of the telephone (1) can not be detected by the absolute value of the loop current because of the non-uniformity of the ine impedance. It has been necessary to detect it by the relative increase which causes unreliable operations.